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We have found that StellarisWare and TivaWare use a USB mode that is not compatible with the EK-TM4C123GXL (Tiva C LaunchPad).
The work round is rather simple. You must change the default mode of the USB stack to disable VBUS monitoring which is not connected in hardware on the EK-TM4C123GXL.
Full details and work around can be found at http://processors.wiki.ti.com/index.php/Tiva_C_USB_Mode_Force_Device.
This issue should be addressed in the release of TivaWare that follows 1.1.
Dexter